Armature X-axis mirror not weighting correctly
This issue is, as you have identified, due to the mirror modifier. Technically speaking there is only one hand in the geometry data and the weight painting system is only reading one hand in two places, because weighting is technically a vertex group assignment system. If you apply the mirror modifier (I suggest making a duplicate of the hand if you aren't sure about needing to make changes later.) it will create the geometry for the mirrored hand. Then you can go back into weight painting and the x axis mirror should work as intended.